Ion mobility spectrometers (IMS) are used for detecting trace substances in the air. They are widely used particularly in the detection of explosives, illegal drugs, chemical weapons and toxic industrial gases. The characteristic structural components of an ion mobility spectrometer are ionization chambers, drift chambers and detectors. The ionization chamber and the drift chamber in conventional ion mobility spectrometers are usually separated by a grid. In the ionization chamber, the analyte molecules to be determined are converted into ions. The ions formed are transferred from the ionization chamber to the drift chamber as an ion swarm due to the effect of an electric field. Under the effect of an electric high voltage field, the analyte ions pass through the drift chamber against the resistance of the drift gas and are, partly due to a different mobility of various ions, detected by the detector in a time-resolved manner, because different analyte ions display different interactions with the drift gas, therefore have different flight times and may thus be separated from one another.
Ion mobility spectrometers have become known, in which the drift gas flows from the detector in the direction of the ionization chamber. The analyte gas is ionized and flows within the ionization chamber in the direction of a grid. The ions formed thus move with the analyte gas in the direction of the grid and then up to the detector under the effect of a high voltage field against the direction of flow of the drift gas (Spangler and Carrico, Int. J. Mass Spectrom. Ion Phys., 1983, 52, 627).
A unidirectional flow guide, in which the analyte gas is fed into the device on the detector side and leaves the device again on the ionization chamber side, is described by Eiceman in U.S. Pat. No. 4,777,363. The ionization takes place in the ionization chamber, and the ions are accelerated against the flow of analyte gas up to the detector. Drift gas and analyte gas are identical here.
Both systems require a homogeneous electric field within the drift chamber for the separation of the ions. This homogeneous electric field is composed from a series of annular electrodes, each of which is electrically insulated. The necessary high voltage is usually 2,000–3,000 V. Such systems are very expensive, complicated to manufacture and are miniaturized only with difficulty.
Furthermore, unlike the IMS described above, it has become known how to guide the ions to be separated unidirectionally with the drift gas flow. The ions can be deflected out of this direction of flow by a relatively low voltage. Once they have then reached electrodes, which are formed by the walls, they can be discharged, and a flow can be measured. Drift gas and analyte gas are identical here.
Such a system is found in so-called electron capture detectors. Lovelock discloses an early example in U.S. Pat. No. 3,870,888. Total ion flows can be measured with such systems. On the other hand, making a distinction between individual types of ions is not possible.
It has become known how to separate long-lived ions from short-lived ions by means of extending the drift sections, e.g., by incorporating flow spoilers. This principle is described, for example, in the detection of chemical weapons (U.S. Pat. Nos. 3,835,328, US 4,075,550, as well as US 5,223,712). The separation efficiency of systems of this type is relatively poor, which may relatively frequently lead to the triggering of a false alarm.
A variant is described by Puumalainen in U.S. Pat. No. 5,047,723. In this case, the gas flow to be analyzed is first ionized and then guided by a series of electric deflecting fields. Depending on the type of the ions, these are each discharged to different electrodes. The flow is measured and is an indicator of the analytes present.
In WO 9416320 Paakanen et al. further modified a system of this type and identify substances based on the characteristic patterns that result from a plurality of electrodes closely connected in series by means of the discharge of ions. Besides ion signals, signals of semiconductor sensors were also included in a pattern recognition.
Furthermore, it has become known how to improve the last-mentioned system by the analyte gas being heated before the analysis and by the sensor electrodes forming multidimensional arrays (US 2003/0155503 A1). In this case as well, the evaluation of signals is based on a pattern recognition. Connected to this is the drawback that the measuring system must first learn the respective pattern, thus an extremely high calibrating expense is necessary. This applies particularly in mixtures. Not considered mixtures, i.e., for example, combinations of analytes to be monitored with unknown impurities, may lead to false alarms or may prevent the detection of the analytes to be monitored.
Finally, it has become known to deflect the analyte ions by means of a high-frequency alternating field, on which is superimposed a low compensating voltage. Here, the analyte ions are transported in a system likewise in the direction of the drift gas (U.S. Pat. No. 6,495,823). A defined kind of analyte ion is allowed through the system and reaches the detector only under the defined conditions of the alternating field and of the compensating voltage. These ion sensors, which can be manufactured structurally small, can be joined together into arrays. Systems of this type are, however, expensive and extremely susceptible to environmental influences, such as pressure and humidity.